This invention relates to the field of waste disposal. In particular, this invention relates to the field of municipal waste disposal, including disposal of municipal solid waste (MSW), refuse derived fuel (RDF), biomass, discarded tires, and sewage sludge. Briquettes which may be produced according to the present invention may be used as landfill or as alternative fuel sources.
The disposal of municipal waste, including both solid waste, or xe2x80x9cgarbagexe2x80x9d, and sanitary waste, or xe2x80x9csewagexe2x80x9d is clearly a major economic concern for western society in the 21st century. Conflicting demands of technique, regulation, and politics create a rich field for invention and innovation. A waste stream emanating from a major urban center may include both municipal solid waste (MSW) and sewage sludgexe2x80x94a semi-solid residual fraction resulting from chemical and physical treatment of septic human wastes in sewage or water pollution control plants requiring eventual disposal in landfill or by deep water dumping. The waste stream may also include pre-separated sub-streams, including for example refuse derived fuel (RDF) and biomass. RDF is a partially separated or processed fraction of solid waste containing a higher proportion of waste paper, wood chips and other combustible materials, and substantially free from metals and inorganics, and thereby suitable for use as a fuel. Biomass comprises disposed plant material, often yard waste or such material cleared from land as a fire preventative measure, and is primarily cellulosic, and often has a high moisture content.
Aims in the disposal of municipal solid waste are, as always, economic, and the various objective to which an economic value may be attached are: reduction in landfill airspace volume used; reduction in landfill leachate or run off; reduction in landfill effluent gasses; production of useable energy from waste; recovery of useable waste fractions from the waste stream; and reduction in air pollution resulting from waste combustion. A xe2x80x9ctraditionalxe2x80x9d method of waste disposal may be characterized as dumping of an undifferentiated waste stream in an open location, which, in a slightly modified form, becomes creation of so-called sanitary landfill, with some effort at ground water protection, compaction, and covering. A traditional method of waste disposal is being phased out in most locations, including the developing world, where large waste reclamation and control projects are being undertaken; albeit a large fraction of global waste is still disposed of in an approximately traditional manner, modified by informal recycling efforts, or xe2x80x9cgarbage pickingxe2x80x9d
In moving from a traditional system to one meeting one or several of the above listed objectives, one encounters the characteristic economic problem of tradeoffs, or conflicting objectives. Notwithstanding, multiple objectives may be simultaneously advanced beyond a level of undifferentiated open disposal of waste, with an optimal mix determined by local conditions, and available technology.
Municipal waste buried in landfill is subject to slow degradation under microbial action, producing effluent gases, as well as objectionable odors. Chief among landfill effluent gasses is methane, which is thought to be a contributor to global warming and ozone destruction. Methane can be collected by post-fitting existing landfill with gas collection systems, and may be evolved in usable quantities over a 50 year period following waste disposal. Burned as fuel, the methane is effectively substituted by carbon dioxide as an effluent, which may still contribute to global warming. It is estimated that only about one quarter of the potentially useable energy of a waste pile can be recovered this way, however, relative to methods employing direct burn, or burn after processing. Gas production does have an advantage of yielding a clean burning fuel; heavy metals and problem compounds are left behind in the ground, in the dump, rather than possibly being injected into the atmosphere on combustion, and requiring scrubbing technology to partially ameliorate.
Conventional waste disposal landfill has a disadvantage of dimensional instability, which, coupled with gas evolution, make such a site unsuitable for reclamation for other use for a period of at least 50 years.
Compared to landfill gasification, a more rapid production of fuel gas from waste is possible by placing the waste in specially designed reactors for the acceleration of bio-fermentation.
A production of pellets and briquettes from solid waste streams and sewage sludge is known in the art, with numerous recipes reciting a mixing of the waste stream with coal or other binders. Existing patents concentrate on a evaluation or characterization of physical-mechanical properties of the resulting briquettes and pellets, and on an ability of these briquettes to undergo waste-to-energy transformation, in particular for production of electric power.
A use of pellets and briquettes as a fuel stuff in a waste-to-energy or refuse-derived fuel (RDF) fired plant was introduced by Schulz in U.S. Pat. No. 4,225,457. Briquettes of specified geometry and composition are produced to serve as feed material or burden in a moving-burden gasifier for a synthesis of fuel gas from organic solid waste materials and coal. The briquettes are formed from a mixture of shredded organic solid wastes, including especially municipal solid waste (MSW) or biomass, and crushed caking coal, including coal fines. A binder may or may not be required, depending on a ratio of Coal/MSW, and compaction pressure employed. Briquettes may be extruded, stamped, or pressed, employing compaction pressures in excess of 1000 psi, and preferably in the range of 2000 to 10,000 psi. A ratio of caking coal to shredded municipal solid waste is selected so that each part of a predominately cellulosic organic solid waste will be blended with 0.5 to 3.0 parts of crushed coal. Suitable binder material include dewatered sewage sludge, xe2x80x9cblack liquorxe2x80x9d rich in lignin derivatives, black strap molasses, waste oil and starch; when used, a binder concentration is preferably in a range of 2 to 6 percent.
In U.S. Pat. No. 4,426,042, entitled xe2x80x9cMethod of Shredding Solid Wastexe2x80x9d, an improved method of shredding MSW or garbage is disclosed, which maintains the moisture content of the waste. The shredded waste is then available for efficient disposal or further processing. In U.S. Pat. No. 4,934,285, entitled xe2x80x9cMethod for Treating Waste Materialsxe2x80x9d, there is described a method for treating waste materials which divides the waste material into a compost fraction and a refuse-derived fuel fraction, wherein the compost fraction is exposed to a biological treatment to produce a gaseous fuel. The refuse-derived fuel is burned at high temperature, and the exhaust gases are routed into an after-burning chamber, along with the gaseous fuel from the biologically decomposed compost.
In U.S. Pat. No. 4,227,653, entitled xe2x80x9cMethod of Processing Waste Materialsxe2x80x9d, moist municipal waste is subjected to a preliminary comminuting action to reduce particle size, and the larger heavy particles are segregated from lighter particles which include relatively small fibrous particles and relatively large additional lighter particles. The relatively large additional lighter particles are then segregated from the relatively small fibrous particles and subjected to a severing action to reduce their size to a size not exceeding that of the small fibrous particles. The fibrous particles are then mixed with the severed additional particles, the mixture is dried and ozonized, and portions thereof are used for the manufacture of shaped articles.
In U.S. Pat. No. 4,540,467 entitled xe2x80x9cMethod of Fragmenting Municipal Solid Wastexe2x80x9d, a method and apparatus is disclosed for the removal of mold core material from metal castings and for fragmentation of municipal waste materials, e.g. paper products. The method involves heating and hydrating the materials within a pressure vessel. In U.S. Pat. No. 4,540,495 entitled xe2x80x9cProcess for Treating Municipal Solid Wastexe2x80x9d, a process for treating MSW material in the presence of moisture is disclosed, aimed at the recovery and separation of inorganic and organic matter. In U.S. Pat. No. 5,190,226 entitled xe2x80x9cApparatus and Method for Separation, Recovery and Recycling Municipal Solid Waste and the Likexe2x80x9d, there is disclosed an apparatus and method for separation and recovery of MSW, via the introduction of solid waste material into a rotatable pressure vessel, which vessel rotates, pressurizes and heats the waste material while simultaneously applying an extruding action. The extruding action is achieved by a rotatable extruder mechanism in the pressure vessel which forces the processed solid waste material through a constricted area adjacent the exit of the vessel.
In U.S. Pat. No. 5,265,979 entitled xe2x80x9cHigh Efficiency Waste Placement System for Municipal Landfillsxe2x80x9d, the solid waste is placed in a preselected geometric form to form a waste pile, followed by coating of the exposed portion of the pile with a synthetic cover compound comprising a liquid, binder, cellulose fibers, and plastic fibers, followed by what is termed xe2x80x9cbiostabilizingxe2x80x9d the pile, and compaction. In U.S. Pat. No. 5,352,710 entitled xe2x80x9cNeither Toxic Nor Environmentally Noxious Foamed-Polymer Productsxe2x80x9d, a cellular or foamed-polymer product is disclosed formed by the combination of a variety of polymer foam-forming compounds. The cellular or foamed polymer products are then said to be safe with regards to health and environment, and therefore disposable as waste similar to municipal solid waste. U.S. Pat. No. 5,361,994 entitled xe2x80x9cApparatus and Method for Preparation and Separation, Recovery And Recycling of Municipal Solid Waste And The Likexe2x80x9d, there is disclosed a pressure vessel for subjecting waste material to heat and pressure while simultaneously applying an extruding action to the solid waste. The extruding action is achieved by a rotatable extruder mechanism, similarly to U.S. Pat. No. 5,190,226.
U.S. Pat. No. 5,797,972 to Schultz discloses a sewage disposal process and product: Mechanically stable pellets or briquettes useful as fuel result from combining a major portion of sewage sludge solids with lesser amounts of lime and binder materials suitable for imparting stability to a pressed or extruded product. Coal may also be included in the pellet or briquette composition, but is not essential. The products contain 50 to 95% dry weight percent sewage sludge solids, with a typical value of around 70%.
As may be seen upon the above review of the prior art, numerous efforts have been explored to improve upon the processing of MSW, with the objective of producing a product that is either densified or converted into a fibrous or particulate form, so that it can be better separated from a commingled waste stream and be made to take up less room in a landfill, or can be burned as fuel with a higher energy output per pound mass of fuel. However, as is also clear from the above review, there are limited reports and emphasis on the fact that MSW itself could somehow serve as a direct substrate for the production of a molded products, if somehow MSW could be modified into a non-toxic and useful solid feed resin-type material.
As discussed above, briquetting is known as a method of compressing a relatively high quality fraction of municipal waste into a form suitable for use as fuel. It is not known nor thought practical to incorporate significant quantities of lower-quality waste streams in a briquetting process for eventual use as fuel, nor to produce, from mixed waste sources, briquettes of sufficient chemical and biological stability for long term disposal by burying without an unacceptable degree of decomposition.
Briquetting is not the same as pelletizing waste. Many waste operator and regulatory agencies in this country are familiar with a technique which produces pellets from waste streams. Pelletizing has not proven to be a practical solution to the problem of waste reduction: In the pelletizing process a final product generally has a moisture content of 20% or more, which results in degradation, fermentation and dimensional instability. Pellets can be used in thermal destruction plants, but can not be stored at a landfill site for an extended period of time without fermentation and expansion in volume.
It is an object of this invention to provide an efficient way of disposing of municipal solid waste, sewage sludge, and admixtures thereof.
It is a further object of this invention to manufacture a product from municipal solid waste and sewage sludge which is volumetrically compact compared to other forms of these materials.
It is yet a further object of the invention to manufacture a product in accordance with the previous objects which is dimensionally stable.
Yet another object of the invention is to manufacture a product from sewage sludge and municipal solid waste which is stable after sustained exposure to precipitation and rainwater.
Still a further object of the invention is to effect a landfill airspace savings through volumetric reduction of municipal-solid-waste-derived material for incorporation in landfills
Another object of the invention is to process municipal solid waste into a form which does not emit detectable levels of offensive odors.
A related object of the invention is to process municipal solid waste into a form which will not produce significant gaseous by-products of fermentation and decomposition under conditions encountered in a landfill.
A further related object of the invention is to produce a refuse derived fuel which may be stored for extended periods of time or transported without unacceptable decomposition.
Any one of these and/or other objects of the invention may be readily gleaned from a description and illustrations of the present invention as described herein.
The invention relates to a process for the production of mechanically stable and biologically inactive briquettes or pellets suitable to be used in waste-to-energy processes, or to be disposed at landfill sites for enhanced air volume conservation. A key to the successful features of this process is an end product which is aseptic, and will remain biologically inert for decades under conditions encountered in a landfill facility.
Mechanically and biologically stable briquettes and pellets are obtained from either municipal solid waste (MSW), refuse derived fuel (RDF) or municipal sewage sludge and their combination for purposes of disposing said briquettes and pellets as fuel in waste-to-energy processes or to form geometric aggregates with such briquettes for their disposal at landfill sites. A use of binding material is not required, and whereas fuel additives such as crushed coal and petroleum residues may be added to enhance fuel performance, these additives are not needed to improve waste processing or product stability or mechanical properties. An absolute lack of any required binding material is a relevant factor establishing a useful novelty of the instant process over those otherwise described in the prior art, such as U.S. Pat. No. 5,797,972 to Schultz or U.S. Pat. No. 3,910,775 to Jackman.
In summary, a waste processing method in accordance with the present invention involves the following steps:
(i) reduction of a distribution of particle size by operation of at least one hammer mill shredder;
(ii) passage of the resulting fluff through a rotating kiln dryer, which also effects a separation into heavy or inert (metal and inorganic) portions, and an airborne portion; and
(iii) following removal of exhaust gasses in a cyclonic separator, feeding of substantially dried fluff into a twin screw extrusion machine for production of extrudate.
Other steps include scavenging of ferrous metals, screening, and cutting of extrudate into uniform sized blocks. An additional major improvement of the present invention with respect to the prior art is an ability to process a plurality of waste streams, such as MSW, RDF, tires and sewage sludge, by invariably producing briquettes or pellets with superior mechanical properties and biological features, without a use of any binding material or additive.
The present invention also relates, in one of its many aspects, to a process of producing briquettes and pellets of suitable shape and dimensions to be arranged (stacked) in geometric aggregates which facilitate a disposal of such briquettes and pellets at landfill sites by at the same time introducing a significant savings in transportation cost to a landfill site and substantially reducing an environmental and social impact associated with transportation of MSW and sewage sludge from a production or collection site to a landfill or other disposal site. The invention also relates to a method of controlling final residual moisture and density, by varying, among other parameters, a size of waste material after shredding, an extrusion speed, a selection of extrusion die, a cut-off dimension and thereby, indirectly, a temperature profile inside an extrusion equipment, in order to obtain products of desired properties, according to a final market application or destination.
The general aim of a process in accordance with the present invention is to resolve the shortage of landfill space experienced in densely populated areas of the country, by volume reduction of the waste by compaction, and possible diversion of waste to energy applications.
According to another central aspect of this invention, a typical municipal solid waste stream input density in a range of 400 to 600 lb/yd3 may be increased at an output of a waste processing plant in accordance with the present invention to between about 1,000 and 3,000 lb/yd3, preferably 2,000 to 3,000 lb/yd3, more preferably 2,000 to 2,400 lb/yd3; a 4 to 5-fold improvement in packing density or compression. This enhancement of density when combined with a method, also introduced in connection with the present invention, of assembling processed briquettes in packs and/or geometric formations with a bulk density close to that of isolated single briquettes usually yields landfill airspace volume usage two to three times lower than that obtained at landfills by compacting MSW. In other words, a landfill employing the present methods may be expected to achieve a packing factor of at least twice that obtainable with existing technology of dumping of an uncompacted solid waste stream followed by an in situ compaction utilizing heavy earth-moving equipment.
To achieve claimed densities, briquettes must be disposed in a uniform geometric pattern, rather than merely tumbled, into the landfill. A close packed pile of briquettes occupies approximately ⅔ a volume of an equivalent mass of randomly piled briquettes; so an additional volume reduction factor of at least about ⅓, in addition to reductions resulting from waste compression, may be achieved by close packing. In practice, the briquettes are packed and strapped in uniform blocks for transport, of the order of 1 to 3 cubic yards in volume. These blocks may be packed at a density not less than 90% that of individual briquettes, allowing for irregularities and packing errors, owing to an ability to produce essentially rectangular briquettes via a choice of exit die on an extrusion machine, and a perpendicular cutoff saw. The blocks may be handled by clamping at a landfill site, with final positioning and packing by heavy equipment. Blocks gain in handling strength over a straight stacking via packing in an xe2x80x9cEnglish bondxe2x80x9d or xe2x80x9ccommon bondxe2x80x9d pattern, as is known in brick-laying, and will be described in detail herebelow.
It is also possible and advantageous in some applications to produce extruded product of an intermediate density range, of between 1,000 and 1,400 pounds/cubic-yard, for use for example in cement plants, where a lower density fuel is required and there is no requirement for biological inactivity. In general a requirement for biological inactivity is relaxed in fuel applications as compared to use in landfill, except insofar as an aseptic condition improves shelf-life and limits odor problems in an application where fuel is not to be burned nearby, with a short inventory turnover. In general it is found that a final density of 1,600 pounds/cubic-yard or more is required for a biologically inactive product, which can be landfilled or possibly otherwise stored for subsequent use as fuel.
An ability of the process of the present invention to handle commingled waste streams at design capacity of the plant may be partially hindered by a percentage composition of a composite stream. In particular, preponderant amounts of sewage sludge (above 30%) may require the introduction of a modified kiln in order to maintain a quality of output of the plant within design required output parameters. In this case a waste processing plant in accordance with the present invention includes 2 separate kilns; dried wastes from these two kilns are then mixed before entering the briquetting machine.
Independent of a relative amount of sewage sludge, partially dewatered in a filter press or in a belt press, a maximum moisture of 15-20% should be attained in the mass of sludge before this can be fed into the extruder. Heat necessary to evaporate excess residual sewage sludge moisture may be obtained by burning landfill biogas, if the plant is operating near or at a landfill site, natural gas, fuel oil, or a portion of the briquettes produced by a plant operating in accordance with the present invention. In case a thermally activated second sludge drying stage is contemplated, an acid-basic solution scrubber is ideally used followed by an ozone treatment unit, to eliminate possible noxious odors in an exhaust gas plume from a treatment plant operating in accordance with the present invention.
A method for processing a composite municipal waste stream with a method in accordance with the present invention contain three main elements, as will be laid at greater length in a detailed description section below:
(1) A step employing a rotating kiln, heated by a methane or gas burner, capable of producing a waste intermediate with residual moisture content of about 10%, and also removing ferrous and non-ferrous metal particles and inert materials.
(2) A step employing a twin screw extrusion or briquetting machine capable of executing an extrusion operation further reducing a moisture content of a processed waste stream to approximately 5% while maintaining an extrudate temperature between 250 and 280xc2x0 F., so as to produce an essentially biologically inert or aseptic briquette. The extrusion process does not make use of binders, but rather controls a temperature profile within the extrusion machine in order to obtain desired product characteristics.
(3) A method of forming and assembling the briquettes in geometric packs of different size and shape, and of optimizing a disposal of the packs in landfills, lowering air space requirement by a factor of 50-70%.
Further considering the kiln or dryer, which is an integral part of a best mode the present invention, a preferred design includes several provisions for a safe operation of a waste processing plant, which have been tested at prototype facilities:
A concurrent flow of heated air and material allows the material to be in contact with the hottest air at the time the material contains the highest level of moisture, thus minimizing a risk ignition of paper and plastic, or other materials with a low flash point. In the unlikely event of fire, three valves located on three different water lines will open in sequence. The first two will open automatically to ensure immediate action and the last one opens manually. The opening of the last valve triggers the initiation of the plant shut down sequence.
Further considering the briquetting machine; a front section includes an electric motor with rating of 400 to 1,000 kW, the motor controlled by an inverter and a gear reducer, which in turn assures and controls a proper rate of rotation for the screws. The screws are built in a high wear resistant alloy and have a diameter of 400 mm to 800 mm. When handling Municipal Solid Waste the 400-mm screws are capable of delivering over 4 tons of extrudate ranging 4 to 7 inches in size, while a similar briquetting machine assembled with two 800-mm screws can deliver up to 10 tons of briquettes of a similar size. Smaller briquettes can be produced by the larger machine at a cost of reduced productivity. Thermocouples located inside the machine monitor and facilitate control of a temperature profile of in-process extrudate. The extrudate, coming off the briquettes, slides onto a take-off roller used to control shape and dimensional stability of the still-soft briquettes. Metal guides on both sides of the roller assure the confinement of the extrudate.
The briquettes subsequently harden with cooling and exposure to air, and to complete sizing a cut-off saw, located downstream from a take off roller of each briquetting machine cuts the briquettes at the desired length. The saw comprises a sturdy frame and a system of idle rollers supporting a motorized belt which in turn supports and draws the briquette to the cutting zone. The cutting unit comprises a circular saw mounted on an idle cart, controlled by a photo-cell actuated motorized cam. A vertical movement of the slitter is controlled by a crank-connecting-rod system and a gear reducer.
The process described above provides a cost competitive and environmentally safe alternative for disposing MSW at landfill sites and extends their residual life. Combustion fumes produced by either natural gas or landfill gas are washed by a jet scrubber before release to the atmosphere.
Fume contaminants are mainly composed of particulate matter in amounts ranging 50 to 150 mg/m3. A use of natural gas as a fuel limits emissions of SOx within regulatory norms without specific treatment. The pH of a washing solution employed in the jet scrubber is continuously monitored to identify the presence of unexpected acid or basic contaminants. A dual acidic/basic solution system is available to bring pH values within limits, and is especially needful in a presence of significant quantities of sewage sludge.
Liquid effluents generated by the process, such as leachates coming from drains and condensate originated in the scrubber, are treated before release to a municipal sewer.
Aseptic briquettes (produced by methods according to the present invention) comprise a mixture of at least one component selected from the group consisting of sewage sludge solids, municipal solid waste and refuse derived fuel. Briquettes according to the present invention comprise about 5% to about 100% by weight paper and plastics with a preferred range of about 10% to about 85% by weight paper and plastics, more preferably about 15% to about 75% by weight paper and plastics and in certain cases about 20% to about 50% by weight paper and plastics. These briquettes have the following preferred characteristics:
1) a density ranging from about 500 to about 5,000 lb/yd3, preferably about 1,000 to about 3,000 lb/yd3, even more preferably about 2000 to about 2750 lb/yd3 or alternatively, about 2000 to about 2400 lb/yd3;
2) a moisture content of less than about 10% by weight, more preferably less than about 7-8% by weight, and in certain preferred embodiments, less than about 5% by weight;
3) storable under average daily temperate zone conditions for a period of at least about one year (i.e., without losing a mass percentage greater than about 5%, more preferably without losing a mass percentage greater than about 2% per year) to decomposition processes biologically and chemically inert under conditions encountered in a landfill for a period of at least about 3 years, preferably at least about 5 years, more preferably at least about 20 years and even more preferably at least about 50 years or more.
4) compressive strength ranging from about 500 psi (pounds per square inch) to about 15,000 psi, preferably about 5,000 psi to about 10,000 psi, more preferably about 8,000 psi to about 10,000 psi; and
5) preferably, metals are removed to a level no greater than about 0.3% by weight. In certain alternative fuel aspects of the present invention, the briquette according to the present invention further includes crushed coal and/or petroleum in an amount ranging from about 0.5% to about 50% by weight, preferably about 5% to about 25% by weight.